1. Field of the Invention
The present invention is directed to an electro-acoustic component of piezo ceramic material, such as electric filters, oscillators, resonators and delay elements.
2. Description of the Prior Act
In electro-acoustic components of piezo ceramic material, tolerances of the frequency behavior or of the delay time are essentially defined by the tolerance of the sound velocity. In high-quality lead-zirconate-titanate ceramics (PZT ceramics), a sound velocity tolerance of .+-.0.5% is currently achieved at best. Narrower tolerances can be achieved by balancing the components.
A method for balancing or varying the transit time between an input transducer and an output transducer on a piezo electric substrate of polarized ceramic is disclosed in U.S. Pat. No. 3,710,465. The remanence polarization is varied therein by applying an electrical field. The modified remanence polarization results in a modified elasticity modules and, therefore, in a modified sound velocity. Large-area electrodes, however, must be present on the component for this purpose. Due to the great steepness of the hysteresis loop in the region of the coercive field strengths, the desired, new polarization can only be inaccurately approached with field strengths in the region of the coercive field strengths.
A method for setting piezo electric, monolithic ceramic filters is set forth in German Patent 236 836. The exact setting of important filter parameters therein occurs by eroding electrode material with laser emission. The dependency of the essentially electrical filter characteristics on the dimensions of the metal electrode configuration of the filter lamina is thereby exploited. Optimum and accurately-reproducible filter properties can thereby be achieved in mass production. However, high temperatures are required for eroding the metallic electrode material and the risk of crack formation and of depolarization is thereby present for piezo ceramic substrates.
A further method for setting the frequency-amplitude characteristic of interdigital transducers is disclosed in German Published Application 23 17 761. The effective finger length of the transducers in this techniques is not influenced by modifying the electrode dimensions but is influenced by depolarization of the piezo electric ceramic substrate lying therebelow. The depolarization is achieved by local heating of the ceramic substrate above the Curie temperature with the assistance of focused laser emission.
Crystalline, piezo electric substrates such as, for example, quartz or lithium-niobate are also employed without additional setting given higher tolerance requirements. Lower band widths and higher insertion attenuation must be accepted due to the significantly poorer electro-mechanical coupling in these materials in comparison to piezo ceramic.